Flying is weird, because it seems like it can consist of either being a bird or having the ability to fly – not necessarily both (see: Dodrio, Scyther). In addition to the fact that there’s only one straight Flying-type (and even that in generation five), it might also be important that Flying is almost never the first of two types; there’s no functional distinction at all, but which element gets put first often seems to say something about which one is considered more important to the design (e.g. Water/Rock – Relicanth and Corsola – vs. Rock/Water – Kabutops and Omastar). Noibat and Noivern, only just introduced with X and Y, are the only Pokémon to put Flying first. What’s more, wind powers – which we now associate with Flying-types, and which are Tornadus’ main feature – originally seem to have been connected with Normal, not Flying, because Gust was a Normal-type attack in Red and Blue, while Whirlwind and Razor Wind, the other wind attacks that existed in the original game, still are Normal. I think what all this adds up to is that Flying wasn’t originally considered a type, as such – it acts more like a trait or ability that certain Pokémon possess, which makes sense when you think about it, because that’s what flight is (Dodrio is explained by the fact that, as we know from Missingno., the type was originally called “Bird,” not “Flying”). Tornadus shows that Game Freak’s conception of what the type means has changed significantly since Pokémon began, but some things about this game resist change. Charizard, then, isn’t so much “a Fire/Flying-type” as “a Fire-type who can fly,” while Pidgeot – and this is important when we move into the next half of your question – isn’t “a Normal/Flying-type,” but “a Normal-type who can fly.”
Now, Normal-types. They rely primarily on attacks of pure bodily strength, but without the complex motions and training that allow Fighting-types to overpower them hand-to-hand. As such, they also find it difficult to harm the rigid, reinforced hides of Rock- and Steel-types, and cannot inflict any harm at all on the insubstantial Ghost-types. However, being more aware than most Pokémon of the limitations of physical reality also makes them distant from the world of ghosts and spirits, rendering them invulnerable to harm from that direction. So, what do I think are the defining attributes of the Normal-type?
None. It doesn’t have any.
Normal is the “everything else” type, where we find Pokémon who have powers that either relate to none of the other elements (like Pigeotto’s wind powers, Jigglypuff’s song, or Chansey’s healing abilities), or that encompass all of them (like Ditto’s transformation skill). This becomes difficult when we confront dual-typed Normal Pokémon… but other than Normal/Flying-types, whom we’ve discussed already, how many of those are there in the first three generations? I count only Girafarig, who is something of a special case because his design focuses on his bifurcated nature; his front half has the more cerebral temperament associated with Psychic-types, while his back half is animalistic and has no special powers. It’s not until Bibarel in generation four, then Sawsbuck and Meloetta in five, that this really starts to change, and only now, with the introduction of Diggersby, Heliolisk and Pyroar, and the retyping of Jigglypuff, that non-Flying Normal dual-types have become a significant element of Pokémon’s diversity. What this says about those Pokémon, I think, is that their elemental powers are comparatively less developed than in the case of other Ground-, Electric-, Fire- or Fairy-types, and that they rely as much or more on their non-elemental abilities (sound-related powers are still unaligned, for instance – hence Jigglypuff, Meloetta and Pyroar, who has a sound-based signature move, Noble Roar). It was difficult to give Normal-types abilities related to types other than Flying for a long time because the thing that made them Normal-types was their lack of any such abilities. It’s only fairly recently that they’ve been able to reconcile this with their desire to use a greater variety of Normal dual-types.
